As a result of hitting some festivals this Summer, I have found myself wanting to build a battery powered portable amp so I can join in with jams and the likes without the restraint needing a power point. Most of my gear is battery powered so I'm half way there.

The main things I am looking for are decent volume and battery life. Sound quality is also up there, but I don't mind if the final build has some kind of "characteristic" sound as most of my instruments are home made and I think that will just extend their uniqueness and might make for some interesting recordings by mic positioning etc. (if that makes sense)

I have a pair of 5ohm speakers extracted from a 25 year old Yamaha keyboard which I think would be a good place to start.

I would like to get some advice on amplifier IC's to drive these, I have used LM386 for a number of projects with varying results. I built a mini battery practice amp using the LM386 a short while ago, but the sound is always distorted, a friend pointed out that the input level is probably too loud which would cause this, so any advice on regulating this would be good.

So I'm thinking of using a pair of LM386 - unless anyone could recommend something better?

You can add both a gain control and an input level control on the lm386. For the input level control add a pot wired up as a voltage divider. This will allow you to "tune it in" to the maximum possible level without distortion._________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

I think I did use a voltage divider for my last battery amp - although perhaps I just used a variable resistor at input in line without tying one pin to GND, will have to look to check it out because it would be nice to have a quick fix for that!

Is the LM386 only suitable for 8ohm speakers though? how would a pair of 5ohm's play out? I was thinking about using a pair of LM386 (or a single IC designed for stereo) so I could switch for stereo input if I wanted

put something like a 1 watt 3.9ohm resistor in series with the 5 ohm speaker and it should be fine, I have done that before and the result was fine._________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

The LM386 datasheet has three graphs for Power Dissipation -vs - Output Power for 4, 8, and 16ohm loads. A 3.9ohm resistor will just waste a percentage of the chip's puny output power as heat._________________"I am endeavoring, ma'am, to create a mnemonic memory circuit... using stone
knives and bearskins." -- Spock to Edith Keeler

Have you got a better IC to suggest? The LM386 has been plenty loud enough for my purposes in the past. Also I thought the minimum it could handle was 8 ohms, I suppose it'll just need a heatsink with a lower resistance load?_________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

Ah ok thanks for pointing that out, there are a couple of interesting graphs there that I haven't paid enough attention to before. So would I be right in thinking the graph for the 4ohm load shows less dissipation, and that would be a good thing? I'm guessing in terms of less power wastage (and better battery life?) which is something I definitely want.

I just had a thought, if I am running a pair of speakers, would the two 5ohm speakers have an overall load of 7.5ohms as opposed to 10ohms (I have faint memories of electronics lectures in college that I didn't pay enough attention to! )

Have you got a better IC to suggest? The LM386 has been plenty loud enough for my purposes in the past. Also I thought the minimum it could handle was 8 ohms, I suppose it'll just need a heatsink with a lower resistance load?

I like the LM380. The '386 is loud enough if you're right on top of it, I suppose. I don't think it would need a heatsink until supply V approaches the maximum.

SineHacker wrote:

I just had a thought, if I am running a pair of speakers, would the two 5ohm speakers have an overall load of 7.5ohms as opposed to 10ohms (I have faint memories of electronics lectures in college that I didn't pay enough attention to! Sad )

Ohm's Law can be applied to Impedances as well as resistance, so two 5s in series would be 10ohms, which isn't much more than 8ohms, which seems to be the optimal load._________________"I am endeavoring, ma'am, to create a mnemonic memory circuit... using stone
knives and bearskins." -- Spock to Edith Keeler

they are definitely something to look into, but the lm380 and lm386n-4 both push up the supply voltage requirements, the lm380 might just be manageable with 10v min but the lm386n-4 needs 24v which is going to be a bit of a pain with batteries.

I will order some lm380 ic's and give them a blast along with the lm386's - I saw on a website that the lm380 uses some of its pins as heat sinks, would it be safe to test on a breadboard or am I likely to experience meltage?

Richardc64, if you wouldn't mind just clearing up this bit of theory for me, would there be a difference in load if the speakers were connected in parallel then?_________________aidanrtaylor.co.uk

For the impedance you could use another speaker in series instead of a resistor, less heat more sound.
(Oh, I see that was allready sort of mentioned, still waking up here )
Another thing you could look into (allthough not as DIY as making it yourself completely)
is taking the amplifier out of a set of computer speakers. Those usually work on a low voltage, are small, and can get pretty loud._________________"My perf, it's full of holes!"http://phobos.000space.com/http://www.acidtrash.com/Stickney Synthyards

I would look elsewhere for the amplification, the LM386 is complete and utter shite (to my ears anyway). Maybe use one of the TDA car amplifier chips? A bit more (clean) power.

I was thinking only this morning, anyone with a strong opinion is usually as wrong as they are adamant.
Sure the sound quality is not on par with a hi-fi system or something like a "professional" audio system, but a lot is down to the speakers used and components selected for the circuitry- people often use very low capacitances for the input parts, which act as high pass filters.
On top of all that, we aren't making some high fidelity audio system, look at the first post; the main thing Mr Hacker wants is volume and battery life _________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

I agree with JingleJoe about the crappiness or otherwise of the LM386. I've seen this over and over again on music electronics forums e.g. never use the 555 timer/LM358/LM324 etc. etc. The original circuit requirements are often not considered, just people's personal prejudices against a particular component. I use the LM386 all the time (especially in headphone amps) and it is perfectly adequate for the job. If you're trying to build a hi-fi amp, then that's a completely different matter.

Gary

P.S. he's right about the highpass filtering as well, as he pointed out as a problem on one of my projects.

You cannot post new topics in this forumYou cannot reply to topics in this forumYou cannot edit your posts in this forumYou cannot delete your posts in this forumYou cannot vote in polls in this forumYou cannot attach files in this forumYou can download files in this forum

Please support our site. If you click through and buy from our affiliate partners, we earn a small commission.